This indicates an attack attempt against a heap-overflow vulnerability in
Microsoft Internet Explorer.
The vulnerability is caused by improper bounds checking of an overly long URL when using IE to access a website that uses HTTP 1.1 compression. It could allow attackers to execute arbitrary code on the victim's system.
Microsoft Internet Explorer 6.0 SP1
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the patch, available from the vendor's web site: